The invention relates to a system for measuring a parameter within a closed surrounding, which system comprises and apparatus containing:    a sensor for sensing said parameter and for delivering a sensor signal related to said parameter,    an electronic circuit for processing said sensor signal into a measurement value,    a transponder, connected to said electronic circuit, which transponder after reception of an external activation signal activates the electronic circuit and the sensor and thereafter transmits the measurement value supplied by the measuring circuit,    a scanning unit for generating and transmitting said activation signal and for receiving said transmitted measurement value, and    a housing in which the sensor, the electronic circuit and the transponder are installed.
A system of this type is known from EP-0,563,713. This prior art system is especially destined to measure a parameter in a closed environment such as the pressure inside the tube. The actual measuring system is contained in a closed package obtained by molding all the various components into a suitable encapsulating material. The housing does not have any connecting means at the outside because all communication with the circuit inside the housing is done through the antenna. The housing can be made very small and can be mounted very easily within a tire or vehicle to measure the pressure inside or mounted within a moving section of a vehicle shock absorber to sense accelerations experienced by the shock absorber as the vehicle to which the shock absorber is mounted travels over a rough road.
A further prior art system is known from U.S. Pat. No. 5,298,894. This prior art system is especially destined to measure a parameter of the fluid in a buried conduit. A part of the conduit is removed and replaced by a housing comprising the sensor and eventually electronics for amplifying the parameter dependent signal delivered by said sensor. This housing is located into a metal subsurface pit enclosure which is closed by a metal pit lid. A transponder is installed into a further housing which is integrated into the pit lid such that the antenna of the transponder is located outside the metal lid to ensure proper functioning thereof.
Other apparatuses for various parameter are for instance embodied as pressure transmitter for measuring pressure, temperature transmitters for measuring temperature, flow meters for measuring flow in gasses or fluids, pH meters for measuring the acidity of gasses or fluids, conductivity meters, viscosity meters etc. In these known apparatuses the electronic circuit supplies a signal to two external connecting terminals. After installation of the apparatus at the location where the measurements have to be carried out these connecting terminals are through cables connected with for instance a display which may form part of a monitoring panel or with a processor for the processing for the measuring signals. The cables could have a significant length. In this manner a number of parameters derived from different locations within a plant, such as temperature, pressure, conductivity, flow, pH etc., can be monitored remotely and preferably at a central location within a plant where each parameter is measured by a separate device.
In many cases it is not necessary to remotely measure and/or monitor a parameter continuously. In many cases it is sufficient to monitor the respective parameter with certain intervals whether or not on a regular basis. In that kind of applications in general one will not use an apparatus comprising a sensor and an electronic circuit but one will use an independently functioning manometer, temperature meter, flowmeter etc. These independently functioning instruments do not have a cable connection to a central post and are therefore often relatively cheap if there are bought. When performing monitoring operations with these independently working measurement instruments in general the measured value during a monitoring operation will be written down or registered in another way and thereafter the collected stored measurement values will be processed any further, for instance by inputting them to a computer. This procedure is rather cumbersome and the invention has now has the object to indicate in which way the writing down of measurement values can be eliminated and the further processing of the measurement values can be automated or at least significantly simplified. A further object of the invention is to provide meters which use transponder technology and can easily replace existing meters which do not use transponder technology.